1. Field of the Invention
The present invention relates to a thin-film semiconductor device, driving circuitry thereof, and an apparatus incorporating the same and using temperature information when operating them.
2. Description of the Related Art
Temperature sensors have been provided on the outside of display devices to permit temperature-dependent correction of the driving waveform for a liquid crystal display. Thermistors, platinum resistance thermometers, thermocouples, temperature sensors with a pn junction diode using bandgap, SAW (surface acoustic wave) temperature sensors, thermosensitive magnetic substances, and radiation thermometers using an infrared ray are mainly used. Temperature sensors with a pn junction diode, which is inexpensive and has a high linearity to temperature, are being widely used. An attempt to substitute light measurement of transmittance or reflectance for the temperature sensor is being made. A photodiode and a photoconductive cell are used for such light measurement.
The prior art temperature sensors have some problems. When a temperature sensor is on the outside of a display device, the indicated temperature of the liquid crystal part is really only an estimated value, since the temperature of the liquid crystal part held between the support substrates is not directly measured. This will be described in detail using FIG. 1. FIG. 1 shows a construction in which two temperature sensors 83 are provided on the outside of a liquid crystal display manufactured by holding a liquid crystal 908 between glass substrates 10 and 29. In the construction, the lower left side of the glass substrate 29 is heated at 100° C., the glass substrate 10 side is at 20° C. and the back surface of the upper right side of the glass substrate 29 is at 35° C. Reflecting the temperature distribution, a temperature distribution of 25 to 65° C. occurs in the liquid crystal 908. On the other hand, the outputs of the temperature sensors are at 20° C. and 35° C. The temperature of the liquid crystal part cannot be correctly identified. The problem is significant especially in a sidelight construction in which the light source of a backlight is arranged at one side. A temperature distribution is different in accordance with a portion of the apparatus in other constructions. Externalizing the temperature sensor from the display makes it difficult to correctly measure a temperature when the liquid crystal is operated.
Including a temperature sensor in a display device has also been considered. In JP2000-338518 (reference 1), a temperature sensing device formed on the same substrate as a thin-film transistor to drive a liquid crystal is used. FIGS. 2A and 2B show equivalent circuits thereof. FIG. 2A shows an equivalent circuit constructed of a thin-film transistor 4 in which a gate electrode is short-circuited with a drain electrode or a source electrode. FIG. 2B shows an equivalent circuit constructed of a thin-film diode 5. In reference 1, a constant-current source located on the outside is connected to both ends of the temperature sensing device to sense a temperature. It is generally considered that using a current source formed on the same substrate as forms the temperature sensor can eliminate any noise problem. When an electric current is constant, a voltage applied to both ends of a diode depends on ambient temperature. This publication shows that a temperature can be sensed from the drain-source voltage. The temperature sensor using a thin-film diode manufactured in a liquid crystal display device measures a temperature which is very close to the temperature of the liquid crystal itself as compared with the temperature sensor outside the display. However, the current source is outside of the device, which is susceptible to noise from external apparatus. The temperature sensor needs to sense a very small electric current of several to several tens of nanoamperes. Thus, lowered accuracy due to externalization of the current source cannot be avoided. Moreover, thin-film semiconductors represented by amorphous silicon, polysilicon, and CG silicon cannot satisfactorily form the pn junction part as compared with a semiconductor using bulk silicon. The reference voltage is easily varied, and the sensed temperatures are varied.
It is generally considered that forming a current source on the same substrate as that of a temperature sensor can solve any noise problem. However, when the current source and the temperature sensor are formed on the same substrate, they have equal current change to temperature change so as to cancel the change in each other. Therefore, it is difficult to sense the temperature change.
Yannis Tsividis has reported in Yannis Tsividis, “Operation and Modeling of The MOS Transistor”, Second edition, WCB/McGraw-Hill, pp. 183-190 that the gate voltage-drain current characteristic of transistors made of bulk silicon exhibits different temperature dependence by a gate voltage. As shown in FIG. 3, as an example, the temperature dependence of a drain current of the transistor made of bulk silicon is hardly seen near a gate voltage of 0.9 V, about twice a threshold value of 0.5 V. In a region lower than the gate voltage, the drain current is higher as the temperature is increased. In a region higher than the gate voltage, the drain current is higher as the temperature is decreased. In bulk silicon, a transistor as a temperature sensor and a transistor as a constant-current source are manufactured on the same substrate. The former is driven in a gate voltage region having temperature dependence. The latter is driven by a gate voltage region having small temperature dependence. In principle, temperature change can be sensed as a voltage.
On the other hand, a semiconductor layer used for a liquid crystal display is of amorphous silicon, polysilicon, or CG silicon, not of bulk silicon. The threshold values are distributed in a wide range and cannot be uniquely determined. Unlike transistors made of bulk silicon, it is difficult to set a gate voltage value based on a threshold value. Temperature monitoring with high accuracy is hard.
In thin-film semiconductor devices, with any of the methods of externalizing a temperature sensor, of locating a temperature sensor inside and having a current-voltage converter outside, or of having both a temperature sensor and a constant-current source inside, it is still difficult to measure the temperature of a liquid crystal with sufficient accuracy for controlling the liquid crystal.